ERIN PARTLAN, DAVID LADNER Clemson University

1. Dissolved CO2 – An Alternative for Cleaning Inorganic Scale from RO Membranes ERIN PARTLAN, DAVID LADNER Clemson University Background Membrane fouling is a major operational issue in reverse osmosis desalination plants. In particular, plants treating brackish surface waters or groundwater can encounter troublesome inorganic scales that cling to membranes and are difficult to remove. While many efforts focus on methods to prevent fouling, it is inevitable in a major plant. Here, dissolved CO2 is proposed as a novel cleaning method to remove scales from inorganically fouled membranes. Materials • CPA2 Hydranautics Low-pressure Reverse Osmosis Membrane • Plate-and-frame membrane cell (GE SEPA) • CaCO3 solution: 200 ppm CaCl2, 200 ppm Na2CO3, 10 g/L NaCl • Silicate solution: 400 ppm CaCl2, 300 ppm SiO2, 10 g/L NaCl Results Calcium Carbonate The best cleaning resulted from dissolved CO2. When cleaned with acid – the standard for removing many types of inorganic scale – similar results were observed. Lastly, clean water provided minimal cleaning. Calcium Silicates None of the attempted cleaning solutions were able to remove silicate scale. Silica is known to be problematic and typically handled with pretreatment to reduce concentration levels. For cleaning, industrial standards vary between acid and alkaline cleaning solutions. Discussion The fact that dissolved CO2 works to remove CaCO3 poses questions about the mechanism by which it cleans. The cell is opaque, so visual confirmation of bubble formation is not possible. Other possible methods by which cleaning can occur include pH effects and reactivity. These are especially true for calcium carbonate scale since the cleaning solution drops to pH 4 after carbonation, and the addition of CO2 itself stimulates changes in carbonate equilibrium. Further work can be done to explore more of these interactions to arrive at the dominant mechanism. Applications Green Alternative Dissolved CO2 can replace conventional antiscalants and/or cleaning solutions. Antiscalants are typically costly and pose a question of disposal. Acidic and caustic cleaning solutions are often prepared on site and require storage of toxic chemicals. Carbon Sequestration For an in-line CO2 application with concentrate disposal through underground well injection, this process could have the added benefit of carbon sequestration. • Procedure • (Clean water flux taken after each step) • Membrane compaction • Reference salt flux • Membrane scaling • Membrane cleaning • Theory • The use of gas for membrane cleaning uses contact between the bubble surface and foulants to shear material from the membrane. Various applications of air flowing over and through membranes have been applied to micro- and ultrafiltration membranes,1 • Dissolved CO2 Cleaning • Carbonate water for cleaning solution. Accomplished by bubbling CO2 into the water column in a vertical pressure vessel. Water volume is 7.5 liters. • Open valve to divert water in pressure vessel through membrane cell. Water exits under headspace pressure until empty, a process which takes less than 10 minutes. Figure 2: Sketch of bench-scale testing apparatus. Figure 4: Comparison of cleaning efficiencies for membranes scaled by CaCO3. Cleaning with dissolved CO2 returned the most flux, followed by acid cleaning. Figure 1: Depiction of gas cleaning mechanism for fouled membranes. Nucleated bubbles form on the membrane and particulates while gas phase bubbles stay in the bulk. The difference with dissolved CO2 is that it is not present as gas until it reaches the membrane. The membrane surface provides nucleation sites for gas bubbles to form. This produces bubbles that are smaller and closer to the scales than any type of two-phase air cleaning. This method was shown to be effective in removing biofilms from reverse osmosis membranes by Vitens Water Technology, a company in the Netherlands.2 Figure 3: An example of a scaling experiment. NaCl does not produce scales and flux decline is linear with increasing concentration. In scaling solutions, flux decreases both due to increasing concentration and scale formation. Figure 5: Comparison of cleaning efficiencies for membranes scaled by silicates. None of the attempted cleaning procedures regenerated flux. References 1. Cui, Z. F., S. Chang, and A. G. Fane. "The use of gas bubbling to enhance membrane processes." Journal of Membrane Science 221.1 (2003): 1-35. 2. Ngene, Ikenna S., et al. "CO2 nucleation in membrane spacer channels remove biofilms and fouling deposits." Industrial & Engineering Chemistry Research 49.20 (2010): 10034-10039. 3. Mi, Baoxia, and MenachemElimelech. "Organic fouling of forward osmosis membranes: Fouling reversibility and cleaning without chemical reagents."Journal of membrane science 348.1 (2010): 337-345. Acknowledgements Clemson University Dept. of Environmental Engineering and Science Contact epartla@clemson.edu